It has been known to measure blood content and also blood oxygenation from the absorption spectrum of skin measured in reflection or transmission. A corresponding methods are extensively covered e.g. in corresponding collections Berardesca 1995, Serup 2006 (see section “further references” at the end of this text) and the references therein.
For example, it has been known to measure blood oxygenation by measuring tissue spectrum with a broad band source [Osawa 1993, Feather 1989, Liu 1995] or at several wavelengths in the visible and infrared spectral range [Nitzan 2009, Diffey 1984, Sinichkin 2002].
More details on a conventional spectroscopic approach for measuring blood content are given in the section “conventional approach” below.